Fix PR30260 - optional<const T> not working.

This patch fixes PR30260 by using a (void*) cast on the placement argument
to placement new to casts away the const. See also http://llvm.org/PR30260.

As a drive by change this patch also changes the header guard for
<experimental/optional> to _LIBCPP_EXPERIMENTAL_OPTIONAL from _LIBCPP_OPTIONAL.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@280775 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/optional/optional.object/optional.object.swap/swap.pass.cpp

@@ -62,6 +62,10 @@ public:
     friend void swap(Z& x, Z& y) {throw 6;}
 };
 
+struct ConstSwappable {
+};
+void swap(ConstSwappable const&, ConstSwappable const&) {}
+
 int main()
 {
     {
@@ -112,6 +116,11 @@ int main()
         assert(static_cast<bool>(opt2) == true);
         assert(*opt2 == 1);
     }
+    {
+        optional<const ConstSwappable> opt;
+        optional<const ConstSwappable> opt2;
+        opt.swap(opt2);
+    }
     {
         optional<X> opt1;
         optional<X> opt2;